Magnetically forcused electron discharge device



May 29, 1956 J. 5. HICKEY, JR 2,748,307

MAGNETICALLY FOCUSED ELECTRON DISCHARGE DEVICE Filed March 6, 1952Inventor: John S.Hi :KeH,JY:

b8 Qjd. 7 His Attor'ne i47.1 i f 4 4 A .44v4 74 v&44 444 United StatesPatent MAGNETICALLY FOCUSED ELECTRON DISCHARGE DEVICE John S. Hickey,Jr., Cohoes, N. Y., assignor to General Electric Company, a corporationof New York Application March 6, 1952, Serial No. 275,155

Claims. (Cl. 313-155) This invention relates generally to electrondischarge devices and, in particular, to electron discharge deviceswherein a magnetic field is employed to focus the discharge current.

A. limiting factor in the application of controllable electron dischargetubes resides in the presence of control electrode current. Controlelectrode current may appear whenever the control electrode assumes apositive potential with respect to the cathode thereby resulting in thecollection of electrons by the control electrode. Control electrodecurrent not only tends to overheat the electrode but also deleteriouslyloads the input circuit connected thereto. Moreover, high values ofcontrol electrode current represent such a substantial portion of theelectron discharge current that the resulting power loss frequentlycauses a circuit utilizing an electron discharge tube to becomepractically inoperative or prohibitively inefficient. the employment ofcontrollable electron discharge tubes are primarily the result of theattraction of electrons to the control electrode when it assumes apositive potential with respect to the cathode.

These and other restrictions upon To overcome the collection ofelectrons by the control electrode in an electron discharge device, ithas been proposed heretofore that the electrons emanating fromthecathode be focused to follow defined paths which traverse theinterstices of the control electrode. Such focusing has been attemptedby the employment of magnetic fields generally aligned with thedirection of flow of the electron current. With such an arrangement, theelectrons having a velocity component normal to the magnetic flux linesare forced into helical paths following the magnetic flux lines and thusare prevented from dispersing and striking the surface of the controlelectrode. However, these magnetic focusing means, while theoreticallyattractive, have not yet found favor as a commercially acceptablesolution to the problem of efiiciently controlling large electron spacecurrents in discharge devices. The chief difiiculty experienced in thisconnection has been that of obtaining a properly shaped magnetic fieldwhich is effective to produce substantial focusing of the electronsemanating from the cathode.

It is, accordingly, a general object of the present invention to providean improved, high current, controllable electron discharge device.

Another object of my invention is to provide an improved means ofmagnetically focusing the discharge current of a controllable electrondischarge tube to minimize collection of electrons by the controlelectrode.

'In accordance with the invention, which may be conveniently describedas incorporated in a high vacuum triode but which should be understoodas being equally applicable to discharge devices having a plurality ofgrids, a static magnetic field is arranged to traverse the space betweenthe cathode and anode through an aperture in the control electrode suchthat the magnetic lines of force or flux lines are substantiallyparallel to the desireddirection of electron flow. The cathode, which2,748,307 Patented May 29, 19 56 is a relatively long electron-emissivemember, is positioned in a groove or slot in the face of one of the polepieces of the magnetic circuit providing the flux. The lines of fluxemanating from opposite sides of the groove or slot in the pole piecetend to converge whereby substantially all the electrons leaving thecathode are very effectively focused into a relatively narrow beam whichpasses through the aperture in the control electrode. This focusing isespecially effective since the electron velocities near the cathode arevery low.

My invention will be better understood from the following descriptiontaken in connection with the accompanying drawings, and its scope willbe pointed out in the appended claims.

In the drawing, Fig. l is a simplified view which illustrates theinvention in general fashion; Fig. 2 is a section view of the electrondischarge tube of Fig. 1; and Fig. 3 is a view taken along lines 3-3 ofFig. 2.

Referring now to the figures of the drawing, there is shown according tothe invention a magnetically focused electron discharge device whichcomprises a controllable electron discharge tube 1 and a magneticcircuit 2. Magnetic circuit 2 may be formed of a permanently magnetized,high coercive force material, such as an ironnickel-cobalt alloy, toproduce a static magnetic field extending axially of the electrondischarge tube 1. To facilitate the production of the desired axiallyextending magnetic field, electron discharge tube 1 is positioned withinthe gap 3 between pole pieces 4 and 5 of magnetic circuit 2. As will beunderstood, magnetic circuit 2 need not be composed entirely of a highcoercive force material but may conveniently include portions of a lowreluctance material such as soft iron. Moreover, magnetic circuit 2 may,if desired, be constructed entirely of soft iron and the requiredmagnetic fiux produced therein by means of a direct current carryingwinding, not shown, positioned about a portion thereof.

Electron discharge tube 1 is represented as a'triode vacuum tube of theplanar electrode type having cathodes 6, a control electrode 7 and ananode 8. Support for the various electrodes of electron discharge tube 1is provided by a plurality of convoluted washers 9, 10, 11 and 12 whichare disk-sealed as shown to concentric cylinders 13, 14 and 15 of amaterial such as glassor ceramic. Cathodes 6 are elongated and arepositioned Within slots or grooves 16 in a pole piece 17 which issupported by disk or washer 12. Cathodes 6 are preferably of theindirectly heated type and include elongated metallic cylinders 18within which conductive heater wires 19 are insulatingly inserted.Cylinders 18 have their upper surfaces coated with a layer ofthermionically emissive material and are attached by anyv convenientmeans such as spot welding. to inwardly extending flanges 29 of disk orwasher 11. Power may be supplied to heater wires 19 through a pair ofconductive leads 21 which are insulatingly introduced into dischargetube 1 through hermetic seals 22 of glass or ceramic material sealedinto disk or washer 12.

Control electrode or grid 7 is formed of a plurality of elongated,spaced-apart, conductive, non-magnetic members 23 suitably attached tothe inner periphery of disk or washer 10 to provide apertures 24 whichare aligned with grooves 16 in pole piece 17 and substantiallycoextensive in cross-sectional area therewith. To provide thuslyapertures 24, elongated members 23 are aligned with bosses orprojections 25 of pole piece 17. Anode 8, a portion or all of which mayserve as a pole piece, is fluted as illustrated to provide bosses orprojections 26 which are aligned with apertures 24 and grooves 16 andwhich are substantially coextensive in cross-section with thecross-sections of apertures 24.

The novel operation and advantages of the invention a can be morereadily understood by reference to the magnetic flux lines 27 which areshown in Fig. 2. For the sake of simplicity of the drawing, flux lineshave been represented only with respect to one of the slots 16; however,it will be understood that similar representations may be made withrespect to the remainder of the slots 16. As illustrated by the fluxplot, magnetic flux lines 27 extend between pole piece 17 and anode 8,passing through apertures 24. The significant aspect of the magneticfield configuration is that the flux lines, fringing out from thegrooves 16 and the adjacent bosses 25 toward anode 8, are notperpendicular to the face of pole piece 17 in the vicinity of grooves 16within which cathodes 6 are supported. Instead, the fringing flux lineson both sides of each groove 16 converge to provide a uniform fluxdensity a short distance away from the pole face of pole piece 17. Thenature of such a configuration is well known and it may be viewed simplyas a characteristic of a magnetic field to align itself to provide asubstantially uniform field. The important effect of the convergentfringing flux lines is that the electrons emitted from cathodes 6 with avelocity other than in a direction perpendicular to the bases of slots16 encounter a forcedue to the magnetic field in which they aretraveling. Such force, as is well known, is perpendicular to both thedirection of the magnetic flux lines and the velocity components of theelectrons perpendicular thereto, whereby the electrons possess helicaltrajectories as they progress along the flux lines to the anode. It isto be understood, of course, that the control electrode 7 or the anode 8or both are maintained at positive potentials during the operation ofelectron discharge tube 1 to provide a relatively strong electrostaticfield which accelerates electrons emanating from cathodes 6 toward anode8. Thus, even those electrons whose initial velocities tend to carrythem beyond the confines of grooves 16 and toward one of thenon-magnetic members 23 of control electrode 7 are forced to follow theconverging lines of flux and pass through apertures 24. Consequently,with a dispersive electron source which emits electrons having initialvelocities of many directions, the electrons are compressed by theconverging flux lines into a relatively narrow beam which passes throughapertures 24 without striking any of the conductive members 23. Sincethe flux lines, after they have converged to form a substantiallyuniform field, remain essentially parallel until reaching bosses 26 ofanode 8, the electrons are assured of following desired paths throughaperturcs 24. The flux lines extending from bosses 25 of pole piece 17and passing through non-magnetic members 23 have substantially no effectupon the electrons emanating from cathode 6 because the more proximateconverging fringing flux constrains the electrons to follow in thedirection of its lines.

It will now be seen that the present invention provides an eflicient anduseful means for the control of high currents without the collection ofgrid current. To increase the available electron current in thedischarge tube 1 multiple cathodes 6 are provided; however, it will beunderstood that a single cathode along with appropriate modification ofthe remainder of the structure may be employed with efficacy if desired.High current densities are readily established by providing a relativelyvery large potential gradient between the cathode and control electrode,it being possible according to the invention to apply a large positivevoltage to the grid without drawing control electrode current sincesubstantially all of the electrons emitted by the cathode are focused bythe magnetic field through apertures 24. The diameters of the helicalpaths of the emitted electrons may be controlled by the respectivemagnetic and electrostatic field intensities so that the pitch is largein proportion to th diameter. For example, in a device constructed inaccordance with the principles of the invention and having a one-halfcentimeter spacing between the cathode a d ntrsl le t o e a m n c ie dha in a that density of the order of 1000 gauss and a 500 volt positivecontrol electrode potential may be employed, whereby the total rotationof the electrons in their travel from the cathode through thecorresponding aperture 24 is less than one radian. With such circuitparameters, the paths of the electrons, while actually being helicalabout the flux lines, may be considered as substantially following theflux lines with the circular orbit being relatively negligible.

it has been found that most effective operation of the apparatus of theinvention is obtained by positioning the uppermost portion of thecathode emitting surface slightly below or coplanar with the surfaces ofbosses 25. The cathodes may be positioned down further within grooves 16but, in such event, the capacitance between control electrode '7 andpole piece 17 is likely to be increased to an objectionable value. Asshown, pole piece 17 is insulated from oathodes 6, thereby permittingthe operation of cathodes. 6 at a slightly positive potential withrespect to pole piece 17 in order that emitted electrons will berepelled from the surfaces of pole piece 17. If desired, cathodes 6 andpole piece 17 need not be insulated from one another and may be operatedat the same potential. Both anode 8 and pole piece 17 may be of a highcoercive force, permanently magnetized material and, in suchcircumstance, magnetic circuit 2 may be composed entirely of soft ironor may be eliminated. Anode 8 may also be composed of soft iron orhighly permeable magnetic material even though pole piece 17 is composedof a permanently magnetized material. There need be no magnetic pole inthe vicinity of anode 3 because, for tubes having very close interelectrode spacings, the ultimate divergence of the flux lines becomes ofdecreasingly less concern with distance from the pole face of pole piece17.

While this invention has been described by reference to a particularembodiment thereof, alternative constructions will readily occur tothose skilled in the art. I, therefore, aim in the appended claims tocover all such alternative embodiments as may be within the true spiritand scope of the foregoing description.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electron discharge device comprising a first magnetic pole piecehaving therein a groove from which a convergent fringing flux extends ina direction generally perpendicular to the base of said groove, acontrol electrode spaced from said first magnetic pole piece, saidcontrol electrode including a pair of elongated non-magnetic conductivemembers having longitudinal axes substantially parallel to the surfaceof said first magnetic pole piece into which said groove extends andbeing transversely spaced apart to provide an aperture aligned with saidgroove, an anode spaced from said control electrode including a secondmagnetic pole piece having a boss opposed to the aperture between saidelongated conductive members and having a cross section substantiallycoextensive with the cross section of the aperture between saidelongated conductive members whereby said convergent fringing fluxextends between said first and second magnetic pole pieces through theaperture in said control electrode, and an elongated cathode disposed insaid groove for producing electrons which are constrained to follow saidconvergent fringing flux through the aperture in said control electrodewithout striking the conductive members thereof.

2. An electron discharge device as defined by claim 1, in which. saidfirst magnetic pole piece is composed of e manently magnetized material.

3. An electron discharge device as defined by claim 1 in which both saidfirst and second magnetic pole pieces are composed of permanentlymagnetized material.

4. An electron discharge device as defined by claim 1,

in which said groove and the aperture between said elongated conductivemembers are substantially coextensive in cross section.

5. An electron discharge device as defined by claim 1, including anevacuable envelope enclosing said anode, control electrode, cathode andfirst pole piece, and magnetic flux generating means located exteriorlyof said envelope and coupled magnetically with said pole pieces forproviding a desired magnetic flux in said pole pieces.

References Cited in the file of this patent UNITED STATES PATENTSHuppert Dec. 1, 1925 Von Baeyer Jan. 20, 1942 Coeterier May 2, 1944Fremlin et a1 Oct. 29, 1946 Hegbar June 27, 1950 Smith June 27, 1950

